The present invention relates generally to lasers, and more particularly, to solid state lasers capable of producing output pulses of very short duration. The lasers of the present invention are particularly useful in producing energetic output pulses, each having a duration in the picosecond range.
Lasers producing picosecond output pulses, or xe2x80x9cpicosecond lasersxe2x80x9d, are useful in many scientific applications. For example, a parametric laser cavity producing high-power, tunable, picosecond pulses, is the most efficient tool for nonlinear-optical studies of narrow-band transitions in the near- and mid-infrared spectral regions. The Nd-based, solid-state laser, having an Nd-doped active medium, is the most common and widely used picosecond laser.
Generally, picosecond lasers, such as the Nd-based, solid-state laser, are constructed in one of two ways in order to generate the energetic, picosecond pulses. In a first example, the laser contains a regenerative amplifier (RGA) for amplifying a seed pulse from about 10 mJ up to the moderate level of 1 to 10 mJ. The laser also contains a power amplifier for boosting the pulse energy further up to about 100 mJ. Such a laser is usually built using a hybrid system consisting of a low-power, diode-pumped, continuous-wave, mode-locked laser, and pulsed, flashlamp-pumped, regenerative and power amplifiers. As this laser combines two different laser platforms, it is both expensive and complicated to use. In the second example, the laser contains a pulsed oscillator for generating a short pulse of 1 to 10 mJ, and a power amplifier for amplifying the pulse power to about 100 mJ. As both the oscillator and the amplifier are built using the same components, operation of this laser is greatly simplified.
The Nd-based picosecond laser described above requires a saturable absorber (SA) with a fast recovery time, or a xe2x80x9cfast SAxe2x80x9d. The fast SA used in most of the previous pulsed picosecond lasers is a dye solution. While a dye solution operates fairly reliably, it must be replaced and maintained on a regular basis. There is a need for an alternative to a dye solution that would not require replacement and maintenance over the lifetime of the laser, such as a solid-state SA.
Progress has been made in the development of fast, semiconductor-based SAs for mode-locking solid-state lasers. However, while these fast, solid-state SAs perform successfully in continuous-wave lasers, they have not been reported as being operable in pulsed, solid-state lasers.
Slow, solid-state SAs can produce ultra-short pulses, provided fast gain depletion or soliton formation occurs in the laser resonator. Unfortunately, these conditions are difficult to obtain in pulsed, flashlamp-pumped Nd3+:YAG lasers.
The Cr4+:YAG crystal has certain characteristics that make possible its use as an SA for passively Q-switching Nd-based lasers. One of these characteristics is its strong absorption band near 1060 nm that allows it to be pumped with an Nd-based laser and to be used as an SA for such a laser. Another of these characteristics is its absorption recovery time of about 8 xcexcsec that makes it a good SA for passively Q-switching an Nd-based laser. For an SA to perform well as a passive mode-locker, however, the SA must have an absorption recovery time similar to, or shorter than, the desired duration of a laser output pulse. Because of its relatively long absorption recovery time, the Cr4+:YAG crystal (a xe2x80x9cslow SAxe2x80x9d) cannot passively mode-lock the laser to produce picosecond pulses.
Thus, the problem of simplifying the pulsed, solid-state, picosecond laser by replacing the fast SA dye solution remains. The Nd3+:YAG picosecond laser using the fast SA dye solution typically produces output pulses of 30 to 40 picoseconds. A negative feedback technique can be used to control pulse duration and energy stability in mode-locked lasers. For example, a passive negative feedback element can be used to shorten the pulse duration of the Nd3+:YAG picosecond laser with the fast SA dye solution to 10 to 15 picoseconds. There remains a need for a pulsed, solid-state, laser for generating short picosecond pulses of stable energy, which laser does not require the fast SA dye solution. Additionally, there is a need for a pulsed, mode-locked, picosecond laser which is simple to use and very stable. It is an object of the present invention to provide such a laser.
The present invention is directed to a pulsed, mode-locked, picosecond laser which is simple to use and very stable. The laser is a solid-state laser having a solid-state laser medium, such as an Nd3+-doped crystal, a saturable absorber (SA), and a passive negative feedback (PNF) element.
In the laser of the present invention, the SA element is xe2x80x9cslowxe2x80x9d, having an absorption recovery time which is longer than a desired duration of an output pulse. Typically, this slow SA would not be capable of operating well on its own as a passive mode-locker. In the present invention, however, the SA and PNF elements together mode-lock the laser to produce an output pulse or pulses of the desired duration. Thus, according to the method of the invention, the solid-state laser medium is energized, whereupon the laser becomes mode-locked to produce the desired output pulses.
Preferably, the inventive laser has an Nd3+:YAG laser medium. The slow SA may be a Cr4+-doped crystal or an LiF:(F2)xe2x88x92 color center crystal, for example, and is preferably a Cr4+:YAG crystal. The PNF element may be a GaAs or a CdSe element, for example, and is preferably a GaAs wafer. The solid-state laser of the present invention is capable of producing very short, energetic output pulses, such as output pulses having a duration on the order of one or more picoseconds, such as from about 1 to about 200 picoseconds, and an energy of from about 100 xcexcJ to about 2 mJ. The laser produces very stable output pulses. As the laser is of relatively simple construction, it is relatively easy to operate to produce this stable energy output.